Thursday, January 04, 2007

Directional Olfaction?

Philosophers tend to take it for granted that our sense of smell is directional or spatial only in a very attenuated sense: We can infer where a scent is coming from (using background knowledge), or we can piece it together by moving about and noticing if the smell is getting stronger or weaker, but at any particular instant a smell is either here or not here, to some degree of intensity. Scents are not fundamentally and intrinsically experienced as coming from a particular direction. (This view goes back at least to Condillac, in his famous example of the statue endowed only with a sense of smell.)

An interesting article, by Jess Porter and others, in the current issue of Nature Neuroscience challenges this view. Subjects were blindfolded, equipped with noise-blocking earmuffs, and given knee- and elbow-pads and workgloves, then asked to track an olfactory trail that bent across a lawn (produced by a string that had been soaked in diluted odorant). They crawled on hands and knees, noses near the ground. Most could in fact follow the trail. (Woof!)

Each nostril draws smell from slightly different regions of space (a fact Porter and colleagues presented as "contrary to the common notion" but which at least used to be widely known among research psychologists -- as discussed, e.g., in Titchener's [1901-1905] Experimental Psychology). This raises the possibility of directional, "stereo" smelling -- discerning the directionality of a scent by the difference between its strength in the two nostrils.

To support this idea, Porter and her colleagues had subjects perform the task with one nostril closed or wearing a "nasal prism device" intended to mask the stereo effect by creating one "virtual nostril". As predicted, these manipulations significantly impaired performance on the scent-tracking task.

Unfortunately, it seems to me that those last results might be explainable by supposing a general decrease in olfactory sensation, rather than a specific cancelling of the stereo effect, since it's not clear that the researchers tested sufficiently whether having one nostril closed or wearing the "nasal prism" impaired olfactory ability generally.

(Thanks to Paul Hoffman, by the way, for the pointer to the Porter article.)

8 comments:

Pete Mandik said...

Nice post, Eric.

One thing that further experiements should look at is what kind of spatial discriminations via olfaction can be effected by stationary subjects. I haven't read Condillac, but I assume that one of the things that would mess up Sniffy the Statue's ability to figure stuff out about space is his inability to move around in it.

So, with both nostrils unplugged, can a subject with body and head immobilized smell where Gandma hid the cookies? How 'bout ol' mono-nostril? My prediction is that two nostrils = yes, one - no.

Eric Schwitzgebel said...

That would be a nice study, Pete! I can't recall anything in the early introspective literature on this (though I haven't surveyed it with this issue in mind), though it's kind of an obvious next question after Titchener's observations about the different fields of smell for each nostril; and if there's any contemporary literature on this, Porter et al. should have cited it.

It's a simple experiment that someone should do!

Jonathan Ichikawa said...

Maybe I'm missing something.

How do these findings contrast with the view that "that our sense of smell is directional or spatial only in a very attenuated sense: We can infer where a scent is coming from (using background knowledge), or we can piece it together by moving about and noticing if the smell is getting stronger or weaker, but at any particular instant a smell is either here or not here, to some degree of intensity"?

People can use smell to follow a ball around; mightn't they be doing it inferentially?

Eric Schwitzgebel said...

I guess my thought was that if Porter's "prism" eliminates the differences in input to the two nostrils without otherwise impairing the sense of smell, and if (as seems to be the case) wearing the prism substantially impairs performance, then directional information as revealed by differences between the input to the two nostrils is being used in the task; and thus maybe the sense of smell is inherently directional in some relatively deep and basic way, integrating "stereo" information at an early, nonconscious level.

But you're right, Jonathan, that there are a few steps in that chain of reasoning where one might want to put on the brakes.

Anibal said...

My own guess is that olfaction works "similar" to other sensory modalities and that the nerve-cell firings of olfactoy receptors in response to molecular cues, and the underlying combinatorial coding made by receptors to produce scents, is finally represented in the brain equal to other topographic representation such as the somatotopic, retinotopic... hereafter to generates a two dimensional space (though in its final outcome resembling an stereo map) with two main axis (intensity= weaker and stronger activations and directionality= the inflows or outflows of molecular cues within air and the skelotomuscular system of creatures equipped with olfaction to approach or avoid the sources of scent).

To distangle issues like the ones Pete referred to, maybe we have to conceive olfaction under the same parameters of other senses, an idea supported by the extreme evolutionary conservatism shown in many cental nervous systems across animal kingdom in order to place, not only spatially but computationally as well, the diferentiated capabilities aided with supplementary functions like memory, past reconstructions, or other cognitive functions.

Eric Schwitzgebel said...

An interesting thought, Anibal! From the little I've seen about the organization of the olfactory cortex, though, it sounds like it's not a spatial organization, with inputs from different regions of space mapping onto different subregions of the cortex (and thus is different from visual and somatosensory cortex in this way). If so, maybe that tells, to some extent, against the idea that olfaction is directional at a very low and early level, or as robustly so as some other senses...?

Porcia said...

I am no scientist, not even college educated, but quite a self-proclaimed geek, if that counts! I just wanted to express how interesting these posts are. My husband & I were having a debate on whether our olfactory senses were directional or not (Ok, fine, I said I could tell where a fart was coming from with my eyes closed and he was laughing at me, but it lead to a mighty geeky conversation about it) Anyway, decided to google it & came across your site. I love all the ideas and perspectives and it is amazing that for something so elementary as one of our senses, there has not been more research done on the subject! I thought I'd add a novice's suggestion to the pot: My father has had only one eye since he was about 4 years old. Now, normally our brains are wired to discern depth using both eyes. That is the way we are made, and if we lose an eye as an adult, depth perception is severely altered. However, since dad's loss was so early on, it seems his brain "re-wired" itself to make up for the change, and his depth perception is much closer to "normal". It seems to me that in addition to experiments mentioned, the way to tell if Anibal's ideas hold up is to find someone (who??? ...there must be a few in the world) who has some deformity or injury since birth or early youth that limits them from using one of their nostrils. This person could be used as a control against others to determine if directional smell is ONLY a case of interpreting the "sterio" of 2 nostrils or not. The plugging of one nostril does not identify with certainty how our brains are able to interpret the smells that come into our nose any more than the covering of one eye can determine the way our brains connect with sight to produce a perception of depth. Of course, I realize there are many other factors involved to get truly scientific data, but this is just one thing to consider. If nothing else, it will give some "real" geeks something to laugh about! ha ha. Thanks for taking the time to read this post. I will be anxious for feedback.

Eric Schwitzgebel said...

That's a cool suggestion, Porcia! If I had a psychophysicist collaboratory and easy access to a group of one-nostriled people, I'd be tempted to do it. Of course, it's exactly this sort of practical barrier that explains why so many interesting questions remain insufficiently explored.